AP Chemistry
Mr. Trubic
St. John’s College High School
AP Midterm Review Packet 2
Answer Key
Name __________________________________
Date ___________________
Directions: Read each question carefully and write your response in the space provided following each question.
Your responses to these questions will be scored on the basis of the accuracy and relevance of the information
cited. Explanations should be clear and well organized. Specific answers are preferable to broad, diffuse
responses. For calculations, clearly show the method used and the steps involved in arriving at your answers. It is
to your advantage to do this, since you may obtain partial credit if you do and you will receive little or no credit if
you do not. Pay attention to units and significant figures.
1. Use principles of atomic structure and/or chemical bonding to explain each of the following. In each part,
your answer must include references to both substances.
(a) The atomic radius of Li is larger than that of Be.
Both Li and Be have their outer electrons in the same shell (and/or they have the same
____________________________________________________________________________________
number of inner core electrons shielding the valence electrons from the nucleus).
____________________________________________________________________________________
However, Be has four protons and Li has only three protons. Therefore, the effective
____________________________________________________________________________________
nuclear charge experienced by the valence electrons is greater in Be than in Li, so Be
____________________________________________________________________________________
has a smaller atomic radius.
____________________________________________________________________________________
____________________________________________________________________________________
(b) The second ionization energy of K is greater than the second ionization energy of Ca.
rd
The second electron removed from a K atom comes from the 3 energy level. The
____________________________________________________________________________________
th
second electron removed from a calcium atom comes from the 4 energy level. The
____________________________________________________________________________________
rd
electrons in the 3 energy level are closer to the nucleus, so the nuclear attraction is
____________________________________________________________________________________
th
much greater than for electrons in the 4 energy level.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Page 1 of 7
AP Chemistry
Mr. Trubic
St. John’s College High School
AP Midterm Review Packet 2
2. Hydrogen gas burns in air according to the equation below.
2 H2(g) + O2(g) → 2 H2O(ℓ)
(a) Calculate the standard enthalpy change, Δ H °298 , for the reaction represented by the equation above.
(The molar enthalpy of formation, Δ H °f , for H2O(ℓ) is −285.8 kJ/mol at 298 K.)
o
ΔH298 = #$2(−285.8 kJ)%& − #$2(0) +1(0)%&
= − 571.6 kJ
(b) Calculate the amount of heat, in kJ, that is released when 10.0 g of H2(g) is burned in air.
(10.0 g H )!#
2
$! 285.8 kJ
&#
#
&#
" 2.016 g H2 %" 1 mol H2
1 mol H2
Page 2 of 7
$
& = 1.42 ×103 kJ
&
%
AP Chemistry
Mr. Trubic
St. John’s College High School
AP Midterm Review Packet 2
(c) Given that the molar enthalpy of vaporization, Δ H °vap , for H2O(ℓ) is 44.0 kJ/mol at 298 K, what is the
standard enthalpy change, Δ H °298 , for the reaction 2 H2(g) + O2(g) → 2 H2O(g) ?
2 H2(g) + O2(g) → 2 H2O(ℓ)
−571.6 kJ
2 H2O(ℓ) → 2 H2O(g)
+2(44.0 kJ)
_____________________________________
2 H2(g) + O2(g) → 2 H2O(g)
−483.6 kJ
(d) A fuel cell is an electrochemical cell that converts the chemical energy stored in a fuel into electrical
energy. Some fuel cells use butane gas, C4H10 , rather than hydrogen gas. The overall reaction that occurs
in a butane fuel cell is 2 C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O(ℓ). What is one environmental
advantage of using fuel cells that are based on hydrogen rather than on hydrocarbons such as butane?
Hydrogen fuel cells produce only water as a product, unlike fuel cells that use
____________________________________________________________________________________
hydrocarbons, which release carbon dioxide. Carbon dioxide contributes to global
____________________________________________________________________________________
warming via the enhanced atmospheric greenhouse effect.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Page 3 of 7
AP Chemistry
Mr. Trubic
St. John’s College High School
AP Midterm Review Packet 2
3. Hydrazine, N2H4, reacts in air according to the equation below.
N2H4(ℓ) + O2(g) → N2(g) + 2 H2O(g)
ΔH ° = − 534 kJ/mol
(a) Is the reaction an oxidation-reduction, acid-base, or decomposition reaction? Justify your answer.
(If you begin your answer with “Yes” or “No,” Mr. Trubic will be very disappointed.)
The reaction is an oxidation-reduction reaction. The oxidation state of N changes from
____________________________________________________________________________________
−2 to 0, while that of O changes from 0 to −2.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
(b) Predict the sign of the entropy change, ΔS, for the reaction. Justify your prediction.
The entropy change for the reaction is expected to be positive. There are three moles
____________________________________________________________________________________
of gas produced from one mole of liquid and one mole of gas. The net increase of two
____________________________________________________________________________________
moles of gas results in a greater entropy of products compared to the entropy of the
____________________________________________________________________________________
reactants.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
(c) Indicate whether the statement written in the box below is true or false. Justify your answer.
The large negative ΔH ° for the combustion of hydrazine results from the large
release of energy that occurs when the strong bonds of the reactants are broken.
The statement is false on two counts. First, energy is released when bonds are
____________________________________________________________________________________
formed, not when bonds are broken. Second, the large negative ΔH° indicates that
____________________________________________________________________________________
the bonds in the reactants are relatively weak compared to the bonds in the
____________________________________________________________________________________
products.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Page 4 of 7
AP Chemistry
Mr. Trubic
St. John’s College High School
AP Midterm Review Packet 2
MgO(s) + 2 H +(aq) → Mg 2+(aq) + H2O(ℓ)
4. A student was assigned the task of determining the enthalpy change for the reaction between solid MgO and
aqueous HCl represented by the net-ionic equation above. The student uses a polystyrene cup calorimeter
and performs four trials. Data for each trial are shown in the table below.
Trial
Volume of
1.0 M HCl
(mL)
Mass of
MgO(s) Added
(g)
Initial Temperature
of Solution
(°C)
Final Temperature
of Solution
(°C)
1
100.0
0.25
25.5
26.5
2
100.0
0.50
25.0
29.1
3
100.0
0.25
26.0
28.1
4
100.0
0.50
24.1
28.1
(a) Which is the limiting reactant in all four trials, HCl or MgO ? Justify your answer.
(0.1000 L)!# 1.0 mol HCl $&
#
"
1.0 L
& = 0.10 mol HCl
%
(0.50 g MgO)!#
1 mol MgO $&
#
& = 0.012 mol MgO
" 40.30 g MgO %
By the stoichiometry of the equation, 0.024 mol HCl is needed to react
with the MgO ∴ HCl is in excess and MgO is the limiting reactant.
OR
The temperature change depended on the amount of MgO added,
indicating that MgO was the limiting reactant.
(b) The data in one of the trials is inconsistent with the data in the other three trials. Identify the trial with
inconsistent data and draw a line through the data from that trial in the table above. Explain how you
identified the inconsistent data.
Trial 1 is inconsistent.
____________________________________________________________________________________
The temperature change should be directly proportional (approximately) to the
____________________________________________________________________________________
amount of the limiting reactant present. The ratio ΔT ⁄ (mass MgO) should be
____________________________________________________________________________________
constant. In trial 1, the ratio is one-half of the ratio in trials 2, 3, and 4.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Page 5 of 7
AP Chemistry
Mr. Trubic
St. John’s College High School
AP Midterm Review Packet 2
For parts (c) and (d), use the data from one of the other three trials (i.e., not from the trial you identified in
part (b) above). Assume the calorimeter has a negligible heat capacity and that the specific heat of the
contents of the calorimeter is 4.18 J/(g·°C). Assume that the density of the HCl(aq) is 1.0 g/mL.
(c) Calculate the magnitude of q, the thermal energy change, when the MgO was added to the 1.0 M
HCl(aq). Include units with your answer.
qcalorimeter = qcal = mcΔT
("
1.0 g
In trial 2: qcal = *$$100.0 mL ×
*)#
1 mL
+"
%
%
' + 0.50 g-$ 4.18 J ' 4.1 oC = 1700 J
'
-,$# 1 g ⋅ oC '&
&
("
1.0 g
In trial 3: qcal = *$$100.0 mL ×
*)#
1 mL
+"
%
' + 0.25 g-$ 4.18 J
'
-,$# 1 g ⋅ oC
&
%
' 2.1 oC = 880 J
'
&
("
1.0 g
In trial 4: qcal = *$$100.0 mL ×
*)#
1 mL
+"
%
' + 0.50 g-$ 4.18 J
'
-,$# 1 g ⋅ oC
&
%
' 4.0 oC = 1700 J
'
&
(
)
(
)
(
)
Points are earned for a correct calculation using any one of the above three trials.
(d) Determine the student’s experimental value of ΔH ° for the reaction between MgO and HCl in units of
kJ/molrxn.
Assuming that no heat was lost to the surroundings, qrxn = −qcal.
o
In trials 2 and 4: ΔH =
o
ΔH =
(
qrxn
nMgO
−1700 J
0.50 g MgO # 1 mol MgO
%
%
$ 40.30 g MgO
)
# 1 kJ
%
& %$ 1000 J
(
(
'
= −140 kJ molrxn
o
In trial 3: ΔH =
o
ΔH =
(
qrxn
nMgO
−880 J
0.25 g MgO # 1 mol MgO
%
%
$ 40.30 g MgO
)
# 1 kJ
%
& %$ 1000 J
(
(
'
= −140 kJ molrxn
Page 6 of 7
&
(
(
'
&
(
(
'
AP Chemistry
Mr. Trubic
St. John’s College High School
AP Midterm Review Packet 2
(e) Enthalpies of formation for substances involved in the reaction are shown in the table below. Using the
information in the table, determine the accepted value of ΔH ° for the reaction between MgO(s) and
HCl(aq).
Substance
Δ H °f (kJ/mol)
MgO(s)
− 602
H2O(ℓ)
− 286
H +(aq)
0
Mg 2+(aq)
o
o
− 467
o
ΔH = ∑ nprodΔH f, prod − ∑ nreact ΔH f, react
= $%− 467 kJ + −286 kJ &' − $%− 602 kJ + 0&'
(
)
= −151 kJ molrxn
(f) The accepted value and the experimental value do not agree. If the calorimeter leaked heat energy to the
environment, would it help account for the discrepancy between the values? Explain.
Yes. The experimentally determined value for ΔH° was less negative than the
____________________________________________________________________________________
accepted value. If heat had leaked out of the calorimeter, then the ΔT of the contents
____________________________________________________________________________________
would be less than expected, leading to a smaller calculated value for q and a less
____________________________________________________________________________________
negative value for ΔH°.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Page 7 of 7